1. Field of the Invention
This invention relates to optoelectronic devices, more particularly, to Vertical Cavity Surface Emitting Lasers (VCSELs).
2. Description of the Related Art
Use of optically transmitted signals in communication systems is dramatically increasing the throughput rate of data transfer. In typical network configurations, an electrical signal is converted into an optical signal by either a laser diode or a light emitting diode (LED). The optical signal is transported through a waveguide such as an optical fiber to an optical detector, which converts the optical signal into an electronic one.
A unit can be assembled that incorporates components for performing many of these functionalities into a single module. Such a module may comprise an integrated circuit, one or more light sources such as LED or laser diodes, and one or more optical detectors such as silicon, InP, InGaAs, Ge, or GaAs photodiodes. The optical detector is used to detect optical signals and transform them into electrical waveforms that can be processed by integrated circuitry in the IC. In response, optical signals are output by the light sources, which may be controlled by the circuitry in the IC. The optical detector(s) may be formed on a silicon, InP, InGaAs, Ge, or GaAs substrate while the optical source(s) are included on a GaAs, InGaAs, InP, InGaAsP, AlGaAs, or AlGaAsSb substrate. The integrated circuitry can be incorporated into either or both of the two semiconductor chips. The two chips may be bonded together, using for example, flip-chip or conductive adhesive technology.
In many cases, laser diodes are preferred over LEDs as light sources. The laser diode, for example, provides a higher intensity beam than the LED. Additionally, its optical output also has a narrower wavelength spectrum, which is consequently less affected by dispersion caused by transmission through the optical fiber. xe2x80x9cLaser diodexe2x80x9d is a general term that includes two broad types of semiconductor lasers. The first type of laser diode is an edge-emitting laser that emits light through an edge of an active region that comprises, for example, a p-n junction layer. The second type of semiconductor laser diode is a vertical cavity surface emitting laser (VCSEL).
A typical VCSEL comprises a plurality of layers of semiconductor material stacked on top of each other. A region centrally located within the stack corresponds to the active region comprising a p-n junction formed by adjacent p- and n-doped semiconductor layers. This active region is conventionally interposed between two distributed Bragg reflectors (DBRs), each DBR comprising a plurality of semiconductor layers with thicknesses selected so as to facilitate Bragg reflection as is well-known in the art.
The term xe2x80x9cverticalxe2x80x9d in Vertical Cavity Surface Emitting Laser pertains to the fact that the planar layers comprising the DBRs and the active region, when oriented horizontally, are such that a normal to the planes faces the vertical direction and light from the VCSEL is emitted in that vertical direction in contrast with horizontal emission emanating from a side of an edge-emitting laser. VCSELs offer several advantages over edge-emitting lasers, for example, VCSELs are typically much smaller than edge-emitting lasers. Furthermore, VCSELs produce a high intensity output. This latter advantage, however, can be negated if the emitted beam cannot be effectively captured and transmitted to an external location, e.g., via a waveguide. Typically, an optical coupling element such as a lens must be positioned adjacent to and aligned precisely with the VCSEL in order to achieve efficient optical coupling. This process reduces the cost effectiveness of using VCSELs in many instances, especially when a plurality of VCSELs are arranged in a one- or two-dimensional array.
Another advantage afforded by the VCSEL is increased beam control, which is provided by an aperture that is formed in one or more of the semiconductor layers. This aperture is conventionally formed by exposing the stack of semiconductor layers to water vapor to oxidize one of the layers. Initially outer edges of this semiconductor layer begin oxidizing; however, this oxidation progresses inward until the water vapor can no longer permeate the layer from the sides, wherein oxidation stop. Thus, a central region of the semiconductor layer remains un-oxidized. When the VCSEL is activated, current will flow through this central region and not the through the surrounding oxide barrier. In this manner, the current flow is confined to a small portion of the active layer. Recombination of electrons and holes within this region causes light to be generated only within a small, localized area within the VCSEL. For the foregoing reasons, this aperture and the layer containing it are conventionally referred to in the art as a current confinement layer.
Disadvantageously, controlling the fabrication of the current confinement layer is particularly difficult. Vapor flow rates, temperature, and exposure time are among the many variables that affect the size and quality of the aperture that can be formed. Precise control of the dimensions of the aperture, upon which the size of the beam critically depends, is particularly problematic.
Accordingly, there is a need for improved optical coupling of the output light from the VCSEL to an external light-carrying medium such as waveguides. There is also a need for a more precise process for fabricating the current-confinement region within the VCSEL that largely defines its beam profile.
In one aspect of the invention, an apparatus comprises at least one VCSEL mounted to a fiber optic faceplate, with the other side of the VCSELs mounted to an IC chip. The IC chips may contain logic circuits that are connected to provide signals to the VCSELs. In various embodiments, the VCSELs may be solder bonded, thermo-compression bonded, or electrical connected to the IC chips with conductive adhesive.
Preferably, the apparatus further comprises a substrate layer between the VCSELs and the fiber optic faceplate. The substrate may include at least one aperture formed therein by etching. The aperture permits passage of light from the VCSELs.
The thickness of the substrate layer may range up to approximately 150 xcexcm.
The apparatus may further comprise an optically transmissive etch stop buffer layer interposed between the VCSELs and the substrate layer. The etch stop layer substantially inhibits etching of the VCSELs during etching of the aperture on the substrate layer. The thickness of the etch stop layer is preferably approximately 0.3 xcexcm.
In another aspect of the invention, an apparatus comprises an IC chip, at least one optoelectronic device, and first and second fiber optic faceplates. The at least one optoelectronic device is mounted on one side to said IC chip. The first fiber optic faceplate is mounted to an opposite side of the optoelectronic device. This first fiber optic faceplate comprises a plurality of optical fibers arranged lengthwise parallel to provide optical pathways between the optoelectronic device and a front face of the faceplate. A second fiber optic faceplate can be mounted to the front face of the first fiber optic faceplate. This second fiber optical faceplate preferably and comprises a plurality of lengthwise parallel optical fibers that provide a pathway rotated at an angle with respect to the lengthwise parallel optical fibers in the first fiber optic faceplate. A reflective surface angled with respect to the optical fibers may be employed to couple light into the fibers in the second fiber optic face plate. These optoelectronic devices may comprise Vertical Cavity Surface Emitting Lasers (VCSELs) and/or optical detectors.